There really aren’t any wine-focussed bookstores in Philly, sadly. While not specifically French, the two essential books that anyone should have are the the Oxford Companion to Wine and the World Atlas of Wine.

As for French-only books, the best of them focus on one specific region. For instance, The Wines of Burgundy by Clive Coates M.W. For a good read (versus information-packed) I suggest Adventures on the Wine Route: A Wine Buyer’s Tour of France by Kermit Lynch